The present invention relates to an overcurrent protection device for preventing rechargeable secondary cells, for example, enclosed lead acid cells or enclosed Nixe2x80x94Cd cells, from discharging overcurrent.
As well known, rechargeable secondary cells, for example, enclosed lead acid cells or enclosed Nixe2x80x94Cd cells have been used widely for power sources of various devices, machines, and the like. Usually, if a large current (overcurrent) is discharged at a moment from such a sort of secondary cell, the cell heats up to deteriorate, whereby the lifetime thereof is shortened. Consequently, dischargeable current value is limited in accordance with the capacitance of an individual cell. These kinds of secondary cells have been used for electric vehicles, e.g., electric conveying carts, electric automobiles, or human-driving vehicles with auxiliary power sources. Since necessary power for the electric automobiles varies every moment, it is necessary to prepare secondary cells, of which the capacitance is much larger than a needed maximum current value in order to prevent the cells from discharging overcurrent.
However, these secondary cells, of which the capacitance is large too much, cause other problems: sizes of devices or machines are inevitable to be large and productions of the devices or machines are costly.
Accordingly, it is an object of the present invention to provide an overcurrent protection device for preventing secondary cells from discharging overcurrent, whereby the lifetime thereof is not substantially shortened although the capacitance of the cells is not large.
To resolve the above-described problems, in accordance with the invention stated in claim 1, an overcurrent protection device is capable of being connected to a current controlling device, the current controlling device including a switching means for allowing a current, corresponding to a duty factor of a pulse for a PWM control provided therefor, flows from at least one secondary cell to a motor; current detecting means for detecting the current flowing to the motor and for outputting this detection result; duty factor calculating means for calculating the duty factor, corresponding to a current value instruction, in order to adjust the current flowing to the motor, so that the detection result of the current detecting means is coincident with a target current value which should flow to the motor; and a pulse generating means for generating a pulse for the PWM control having the duty factor calculated by the duty factor calculating means and for providing the pulse for the switching means. The overcurrent protection device being characterized in that it comprises: limit calculating means for dividing a limit current value, which can be discharged from the secondary cell, by the target current value and for outputting the division result as a limit value of the duty factor; and selection means for comparing the duty factor calculated by the duty factor calculating means with the limit value, the selection means providing the duty factor calculated by the duty factor calculating means for the pulse generating means if the duty factor is less than the limit value, the selection means providing the limit value for the pulse generating means if the duty factor is greater than the limit value.
In accordance with the invention stated in claim 2, an overcurrent protection device is capable of being connected to a current controlling device, the current controlling device including a switching means for allowing a current, corresponding to a duty factor of a pulse for a PWM control provided therefor, flows from at least one secondary cell to a motor; current detecting means for detecting the current flowing to the motor and for outputting this detection result; duty factor calculating means for calculating the duty factor, corresponding to a current value instruction, in order to adjust the current flowing to the motor, so that the detection result of the current detecting means is coincident with a target current value which should flow to the motor; and a pulse generating means for generating a pulse for the PWM control having the duty factor calculated by the duty factor calculating means and for providing the pulse for the switching means. The overcurrent protection device is characterized in that it comprises: limit calculating means for dividing a limit current value, which can be discharged from the secondary cell, by the detection result of the current detecting means and for outputting the division result as a limit value of the duty factor; and selection means for comparing the duty factor calculated by the duty factor calculating means with the limit value, the selection means providing the duty factor calculated by the duty factor calculating means for the pulse generating means if the duty factor is less than the limit value, the selection means providing the limit value for the pulse generating means if the duty factor is greater than the limit value.
In accordance with the invention stated in claim 3, the duty factor calculating means calculates the duty factor at first intervals, and the limit calculating means calculates the limit value at second intervals, the second intervals being prescribed longer than the first intervals.
According to the present invention, the limit calculating means divides the limit current value, which can be discharged from the secondary cell, by the target current value which should be supplied to the motor or by the detection result of the current detecting means. Then, the limit calculating means outputs the division result as a limit value of the duty factor. The selection means compares the duty factor calculated by the duty factor calculating means with the limit value calculated by the limit calculating means. The selection means provides the duty factor, which was calculated by the duty factor calculating means, for the pulse generating means if the duty factor is less than the limit value. On the other hand, if the duty factor is greater than the limit value, the selection means provides the limit value for the pulse generating means. Accordingly, the duty factor of the generated pulse for the PWN control is not in excess of the limit value, so that it is prevented that the overcurrent in excess of the limit of the battery current is discharged from the secondary cells.